Magnetic impulse rapper



2 Sheets-Sheet 1 @TTG m. .f d mm, L mm E 10 T@ y W m 7 5 Mm 7 y wim mm Jf//v// /f/f L May 23, 1961 Filed March 28, 1958 May 23, 1961 J. W`DRENNING 2,985,802

MAGNETIC IMPULSE RAPPER Filed March 28, 1958 2 Sheets-Sheet 2 UnitedStates Patent O MAGNETIC IMPULSE RAPPER John `W. Drenning, Baltimore,

Md., assignor to Koppers Company,

This invention relates to electrostatic precipitators having impulserappers for the electrode assemblies of the precipitators, and moreparticularly to a magnetic rapper for such precipitators.

In electrostatic precipitators electrodes are suspended in aprecipitator shell and exposed to a prevailing particleladen gas streamwhich liows through the shell under pressure. By means of suitableelectrical connections some of these electrodes are positively chargedand some are negatively charged so as to create an electrical fieldwhich ionizes the gas ilowing through the shell. By means of thiselectrical field charges are imparted to the particles and the chargedparticles are attracted to the electrodes having an opposite charge sothat the particles cling thereto with great tenacity. Primarily theseparticles are attracted to the electrodes which are positively chargedbut some particles also are attracted to the negatively chargedelectrodes. The gas, free of particles, then flows out of theprecipitator. In time, the particles tend to accumulate on the suspendedelectrodes to such an extent as to reduce greatly the eiciency of theprecipitator and to weigh heavily the electrodes and their supportingstructures. Periodically, the electrodes are given a series of sharpblows to jar or vibrate loose the accumulated material and loosenedaccumulated material falls of its own weight downwardly to a hopper atthe bottom of the precipitator shell.

Conventionally mechanical and pneumatic impulse rappers have been usedto deliver these blows both to the positively and negatively chargedelectrodes, but an electromagnetic impulse rapper has been found to bemore advantageous for this purpose because the magnitude of the blowimparted to the electrode by the electromagnetic rapper can be so easilyadjusted to the proper magnitude for cleaning the electrodes. Theheretofore known electromagnetic rappers, however, have beenundependable or have required costly and frequent maintenance, such asby the frequent addition of 4lubrication thereto or the elimination ofdirt therefrom. Also the presence of moisture in the rapper environmentas well as the dust particles has caused frequent breakdown in the coilsof the electromagnetic units known heretofore.

An object of this invention, therefore, is to provide an electromagneticrapper which requires infrequent or no lubrication.

A further object is to provide an electromagnetic rapper which is sealedfrom the environment of dirt and dust.

A still further object is to provide a magnetic impulse rapper having amoisture resistant casing for an electromagnetic coil therein.

A still further object is to provide a magnetic impulse rapper which isoperable at elevated temperatures.

This invention contemplates an impulse rapper having a separator and abase adapted to be struck by a hammer to supply vibrations to theelectrodes of an electrostatic precipitator, an encapsulatedelectromagnetic actuator for striking the hammer against the separator,a resilient device for biasing the hammer away from the separator,

2,985,802 Patented May 23, 1961 and a nonmagnetic bearing for guidingthe hammer, the hammer including an arrangement Kfor communicating airfrom one end of the hammer to the other, and a further arrangement forreducing the adverse effects of the electromagnetic actuator on theresilient device.

The above and further objects and novel features of the invention willappear more fully when the same is read in connection with theaccompanying drawings. It is to be expressly understood, however, thatthe drawings are not intended as a definition of the invention but arefor the purpose of illustration only.

Fig. l is an elevational and partial cross-sectional view of the novelrapper of this invention.

Fig. 2 is an isometric view of the electromagnetic actuator of Fig. l.

Fig. 3 is a diagrammatic view of the hammer of Fig. 1.

Fig. 4 is a partial cross-section of an electrostatic precipitator withthe novel rapper of this invention mounted on a supporting structure forthe electrodes.

Electrostatic precipitators are frequently subjected to extremly adverseoperating conditions. For example, the outside of the shell of anelectrostatic precipitator is subjected to air that contains dirt andmoisture and that may have a temperature as high as 200 F. and at thesetemperatures the lubrication conventionally used in magnetic impulserappers tends to become very thin or otherwise ineffective.

An electrostatic precipitator is shown in Fig. 4 as having an electrode11 conventionally in the form of a rod to which are attached wires orplates (not shown), and attached by means of suitable nuts 13 to a plate15 so as to be suspended in precipitator shell 17. The charged particlesfrom a particle-laden gas stream, which is passed through the shell 17,are deposited on electrode 1'1. Plate 15 is supported by a conventionalinsulator 19 which is attached to the outside of the precipitator shell17 by suitable means well known in the art. To the top of the insulator19 is attached a plate 21 in a manner such that the electrode 11 issuspended through a hole 23 in plate 15, a hole 25 in plate 21 and ahole 27 in the precipitator shell 17 which has a diameter substantiallycorresponding to the inside diameter of the insulator 19. Interposedbetween plates 15 and 21 are a plurality of resilient springs 29 whichare arranged so that the electrode 11 is resiliently supported andelectrically insulated from the shell 17.

Housing 31 provides a closed chamber 33 around the top of theresiliently mounted electrode 11 so that there is no leakage of gas fromthe inside of shell 17 to the atmosphere. This housing 31 also protectsthe electrode 11 from weather. Housing 31 is mounted in a conventionalfiuid-tight manner on the shell 17 around the top of the insulator 19and the electrode 11. A projection 35 at one side of housing 31 supportsa conventional ceramic insulator 37, and a conventional synthetic rubbergasket 39, such as a neoprene boot, is supported at the top of housing31. Both members 37 and 39 are attached to the housing 31 in aHuid-tight manner by means well known in the art. A conventionalelectrical connection 41 connected from plate 21 through insulator 37 toa suitable electrical power source (not shown) supplies the necessaryelectrical potential to the electrode 11. A suitable insulating rod 43connected by a conventional metallic coupling 45 to the top of theelectrode 11 extends through the gasket 39 in a fluid-tight manner fromcoupling 45 to rapper 47.

In accordance with this invention, rapper 47 transmits impulses to theelectrode 11 through rod 43 and coupling 45. This rapper comprises ahollow base 49 having a raised portion 51 thereon and a separator 53made of annealled non-magnetic stainless steel mounted on raised portion51 of base 49, a hollow cap 55 attached to the base 49, a magnetic coil57 encapsulated in a casing 59 3 made of a moldable material lsuch as apolyester resin, a bearing 61 in contact with the bore 63 of casing 59,a floating piston 65 having a first portion 67, a second extendedportion 69 and opening 71 for communicating air secured together byfrictional contact but a pin (notv shown) may also be, used through thebase 49 and rod 43. Attached to the base 49 by suitable means such as bywelding is a collar 79. The cap 55, is substantially cylindrical and hasa flange 80 advantageously -weldedv to the cap 55 which is attached tosurface 81 of collar 79 by suitable bolts 83 and nuts 85. A gasket 87between the cap 55 and the collar 79' provides a iluidtight seal. Thus,the inside of cap 55, when the cap 55 is attached to the base 49, iscompletely closed.

The coil 57 may be conventional. In accordance With this invention coil`57 is encapsulated with a moldable material such as a polyester resin.The electrical windings of the coil are placed in a metal mold (notshown), a metal plug is inserted in the bore of the coil, the air iswithdrawn from the mold, and liquid polyester resin, advantageously Acme2001 supplied by the Stone and Barron Company of Baltimore, Maryand, ispoured into the mold. The maximum operating temperature of Acme 2001 isabout 140 F. but the temperature resistance of this resin may beimproved to be as high as 475 F. by usingA a filler such as asbestos.Other resins and illers may. also be used in accordance with suitablemolding processes well known in the art, provided the other resins havethe required molding characteristics, high dielectric strength,dimensional stability, high resistance to moisture, and relatively hightemperature resistance, and where a lubricant is used in the rapper 47for the piston 65, the resin. and ller should also be resistant to suchlubricants. Such other resins include phenolic resinoid,phenol-formaldehyde casting resins, polyvinylidene chloride and othermaterials. This type of molded plastic covered coil has the advantagethat dimensions on the surface ofthe coil covering can be held to closertolerances than was possible with the heretofore known tape coveredcoils, thereby permitting the resin covered coil to iit easily andsnuggly into the cap 55 without elaborate hand adjustments. The moldedplastic covered coil has the further advantage of maintaining a highdielectric.

strength for long periods of time despitey the presence of moisturewhich might en-ter through outlet 89 in the side of cap 55. The moldedcoil has the still further advantage that the coil `57 and lead wires 91are resistant to the shock caused by piston 65 hitting the separator 53during the operation of the rapper 47 and thus the coil windings. andthe lead wires are not `damaged by shock as oftenv happened in the coilsused heretofore.

In accordance with this invention, the top of the molded casing or cover9 has a sill 90 through which lead wires 91 to the coil 5,7 areattached. Thus when the coil 57 is inserted in the cap 55 the lead wires91 are prevented from jamming in the cap 55, jamming of the wires oftenbeingl a problem with the coils known heretofore.

The coil 57 is fitted snugly into the cap 55 so that casing `59` buttsagainst seal 93 at one end and seal 95 at the other end thereby to forma seal between the casing 59 and the cap 55 at one end of thecoil andbetween the casing 59 and the base 49 at the other end of the coil. Bothseals 93 andE 95 `advantageously are made of neoprene but othermaterials such as polytetrauoroethylene maybe used. The bearing 61 isarranged inside the seals '93, andj95` so thatthe bearing member 61contacts the borey63 of casing 59 in a snuggly fitting manner andextendsrfrom base; 49 into recess 9.7y in cap,55. Thus the seals 93, and95, as described above, maintain the inside of bearing 61 free of dirtand dust from the atmosphere and the inside of bearing 61 tight from theescape of lubrication, if present therein.

Bearing `61 is advantageously made of a nonmagnetic material in the formof a hollow cylinder having its inside surface coated with a Tefloncoating 101, Teon being the registered trademark of thepolytetrauoroethylene compound made by the E. I. du Pont de Nemours &Company, Inc., of Wilmington, Delaware. tageously the nonmagneticmaterial but other nommagnetic materials such as nonmagnetic stainlesssteel may be used. This coating 101 substantially reduces frictionbetween the bearing 61 andv the piston 65 as the piston reciprocates inthe bearing thus eliminating the need for lubrication in the rapper.This Teflon coating is advantageously at least .005 of an inch thick. Toobtain optimum results with coating 101, the inside surface ofthebearing 61 should be cleaned initially of any rust,

grease, organic coatings, or dirt, sanded or vapor blasted.

and washed with a volatile solvent. The polytetrauoroa ethylene, as alow-viscosity dispersion in a water medium,

is. applied to the bearing by spraying at room temperature (but it isalso possible to dip the bearing repeatedly until the proper coatingthickness is achieved), and the coating is dried, preferably at roomtemperature belowapproxi-A until sintering of the polytetrauoroethyleneoccurs. It has been found that rap-id cooling of the coating byquenching in cold air or water improves the hardness of the coating andpromotes ease of stripping excess material when necessary. Itisdesirable to use several coatings, it being preferable. to use the abovedescribeddrying andbaking steps between the application of thepolytetrafluoroethylene layers. Each layer should be limited inthickness in order to avoid mud cracking upon dryingv as well as topermit reasonably rapid vaporization of the dispersing agent during thesintering operation.

The Teflon coating 10.1 has a maximum operating temperature range ofover 400 F. and since the other parts of the rapper including the casing59, the seals 93 and 95 have very -high operating temperatures ranges,this no vel rapper can be. used in high temperature locations whererappers known heretofore could not operate satisfactorily.

Inside the bore o'f the bearing 61 is located the floating hammer orpiston 65. The first portion 67 of this piston has a diametercorresponding to the insid'ediameter of the coated bearing 61 and thesecondportion 69 has a lesser diameter. -T he piston is held at the end103` of the cap 55 by the spring 73. Both ends of the piston arechamfered and a hole 71 is drilledr through the center of' the pistonfor the communication of air from one side of the piston to the otherwhen the piston reciprocates. Piston 65 is made of magnetic steel.

Piston 65 is biased away from separator 53Y by spring 73 made of springsteel which is interposed between a conventional solid or split pistonring 107 made of Teflon located on a shoulder 1019 of the piston 65land`a TellenY guide ring 111=1 located on brass spacer ring 75 which is4supported in bearing 61 and on base 49 around' the s eparator 53. Theseparator 53 is force tted inside spacer '75 and located on raisedportion 51 of base 49. It is important that the separator 53 be made ofnonmagnetic material because residual magnetismV exists between thepiston 6,5 and the raised portion 51 of basey 49.- This fo'rce may besuiiicient to retain the piston 6,5y in a downward position if thepiston is allowed to contact the raised portion `51. Separator 53,therefore, being of nonmagnetic material, creates. a gap between` thepiston 65 and the raised portion 51 thus. greatly reduces, the residualforce of' attraction and allows the piston to return freely to the topof cap 55'.A This separator S3 is advantageously made of annealedstainless steel to give it toughness,

Brass is advan otherwise the force of the piston 65 might destroy it.Spacer 75 is advantageously brass but other no'nmagnetic materials maybe used.

The second extended portion 69 of piston 65 extends downwardly throughthe coil spring 73 and the spacer 75. It has been `found that withoutsuch an extension 69 and spacer 75 the spring 73 may be adverselyaffected to such an extent that it will fail after a limited number ofpiston cycles` Surprisingly, therefore, it has been found that byproviding this extended po'rtion 69 and by supporting the spring 73 atone end on the spacer 75 adverse effects are not experienced andconsequently the spring 73 has long life. The reason for this is notcompletely understood but it is theorized that if the spring 73 isallowed to span the air gap between the retracted piston, i.e., when thepiston is at end 103 of cap 55 and the separator 53, the spring being ofa magnetic material, will provide a low reluctance path across the airgap for the field produced by the electrical impulse in coil 57. A stilllower reluctance path is formed if adjacent coils of the spring aretouching. Since a magnetic field will always take the path of leastreluctance, the spring coils are therefore forced to come in contact,each one with its adjacent coil, to form this path. lIt was found byexperiment, for example, that magnetic eld alone, in the absence ofpiston motion, was sufficient to cause coil clash. This coil clash,i.e., the hitting of the coils on each other, flattens the contactingsurfaces eventually leading to spring failure. Thus by placing thespring 73 around the piston portion 69, the path of least reluctance isno longer through the spring but rather through the relatively largermass of magnetic material in the piston portion 69. The magnetic flux isthereby effectively shunted around the spring 73 thus eliminating theforces which cause coil clash. It is theorized that the spacer 75 iseffective to reduce the adverse magnetic effects on the spring 73because the magnetic field is strongest at or near the separator 53 andthus the spacer displaces the spring 73 away from the separator 53 to aposition where the magnetic effects on the spring 73 are less adverse.

`Coil 57 is energized through lead wires 91 which may be attached to asuitable power source through a timer and olf-on switch (not shown).With such timers and switches the coil 57 can be energized for shortperiods of time, and the 'energization may be set at any desirableinterval. An interval between each energization found to be desirable isabout thirty seconds. The energization of coil 57 irnpels piston 65against separator 53.

To change the intensity of the rapper impulse under certain conditionspresent in a precipitator, the voltage maybe increased or decreased by aconventional voltage regulator (not shown) thus resulting in anincreased or a decreased rapper intensity.

In operation, the piston 65 is normally held at end 103 of the cap 55 byspring 73, this being the condition when coil `57 is de-energized. Uponthe energization of coil 57, the piston 65 is impelled downwardlyagainst separato'r 53 to cause a sharp rapping impulse which istransmitted therefrom through base 49 and rod 43 to the electrode 11 ofan electrostatic precipitator. This impulse is substantially constantand repetitive in intensity when the voltage to the coil upon eachenergization is constant. The use of Teflon ring 107 and coating 101eliminates the need of lubrication in this novel rapper because of thelow coeiiicient of friction of Teon. Furthermore the wear of the pistonand Teon parts is negligible so that maintenance is substantiallyeliminated and the impulse intensity remains constant over long periodsof time.

If Teflon coating 101 not be used for bearing member 61 lubrication inthe form of natural or synthetic grease or oil or other lubricatingsubstances such a graphite or molydisuliide is required. The novelrapper operates satisfactorily however, even if lubrication is used andthe rapper be mo'unted on the precipitator shell 17, as shown in Fig. 3,and requires little or no maintenance because the lubrication is sealedin and dirt sealed out as described above. The maximum operatingtemperatures under such circumstances are usually limited by the maximumtemperature allowable with the lubrication and with Gulf Anti-FrictionGrease No. l, for instance, the temperature range is approximately 20 F.to 150 F. A solid Teflon bearing 61 may also be used with goo'd resultsalthough it is more expensive than the last embodiment mentioned.

As shown in Fig. l the separator 53 is force fitted into bearing 61 butseparator 53 may also be provided which is a part attached by suitablemeans such as by bolting to the bottom of piston 65.

The ring 107, cut away, may be located between the ends of the firstportion 67 of piston 465 in a suitable groove as shown in Fig. 3.

The foregoing has described a novel rapper which provides predeterminedrapper impulse intensities, reduced maintenance requirements, infrequentor no lubrication, operation at elevated temperatures, and an interiorsealed from the dust and moisture of its environment.

Although the foregoing has illustrated and described the invention indetail it is to be expressly understood that the invention is notlimited thereto. Various changes can 'be made in the design andarrangement of the parts without departing from the Spirit and scope ofthe invention as will now be understood by tho'se skilled in the art.

What is claimed is:

l. A magnetic impulse rapper particularly adapted for electrostaticprecipitators exposed to adverse ambient conditions comprising a hammer,a base, electromagnetic actuating means having a hollow inside forproducing a magnetic eld for contacting said hammer against said basewhereby vibrations of the base are produced, resilient means Iforbiasing said hammer away from said base, a nonmagnetic spacer interposedbetween said base and said resilient means for reducing adverse magneticeffects on said resilient means whereby said resilient means isoperative within said `actuating means substantially without beingadversely effected by said actuating means, a nonmagnetic bearinginterposed between said electromagnetic means and said hammer forguiding said hammer, and means forming a single closed chamber aroundsaid hammer action and bearing so as to provide a rapper which issubstantially maintenance free.

2. A magnetic impulse rapper particularly adapted for electrostaticprecipitators and exposed to adverse ambient conditions comprising abase for transmitting momentary impulses of high vibrating intensity,electromagnetic actuating means having a hollow core, a sleeve membercoated with polytetrafluoroethylene in the core of said electromagneticactuating means, hammer means reciprocatable in said bearing means, saidhammer means being actuated in one direction by `the energization ofsaid electromagnetic actuating means so as to cause a momentaryhammer-like blow of high intensity to said base, spring actuating meansfor biasing said hammer means in the other direction when saidelectromagnetic actuating means is de-energized and means enclosing saidhammer action in a single chamber so as to separate said chamber fromsaid adverse ambient conditions, whereby said hammer is operativewithout lubrication.

3. A magnetic impulse rapper exposed to adverse ambient conditionscomprising a base having an anvil, a substantially closed housingattached to said base in a fluid tight manner having an outlet therein,a hollow electromagnetic coil in said housing having outlet Wires, andproducing a magnetic field, a molded casing around said coil whichprotects said coil from said ambient conditions and so formed that itsoutside diameter tits snuggly in said housing and so that said outletwires are connected to said coil through said outlet in said housingwithout jamming of the wires, means for sealing the hollow of said coilwith said base and said housing so as to protect the hollow of said coilagainst said ambient conditions, a non-magnetic bearing located in saidmagnetic coil and gest/soa coatedA with polytetrailuoroethylene resin, areciprocatable piston having an upper and lower portion and beingpositioned' in said bearing, said piston having means for communicatingair from one end of said piston to the other end, said piston alsohaving a supporting means thereon extending around its side portion, apolytetraiuoroethylene ring riding on said supporting means, a spacerring in said bearing in contact with said base, and a coil springinterposed between said polytetraliuoroethylene ring and said spacer soas to surround said lower portion of said piston, andV to be removedsubstantially from the strong magnetic tield at said base, wherebyenergization of said magnetic coil impells said piston in one directionagainst said anvil and upon de-energization of said magnetic coil saidspring impells said piston in the opposite direction thereby to providea rapper operable substantially without maintenance and without adverseeffects from said ambient conditions.

4. An electrical precipitator comprising a precipitator, shell, anelectrode assembly resiliently supported on said shell, a rapper housingconnected to said electrode assembly and having a hollow portion closedby a base portion, hammer means in said hollow portion, said hammermeans having a irst portion of one diameter and a second portion oflesser diameter, a brass bearing coated with polytetraliuoroethylene forsaid hammer having an inside diameter substantially corresponding to thediameter of said first hammer portion, a spacer ring in said bearing,electromagnetic actuating means surrounding said hammer, saidelectromagnetic actuating means upon energization, actuating said hammerin one direction so as to cause vibrations in said electrode assembly,and a spring actuating means substantially surrounding said secondportion of said-hammer means and interposed between said `first portionof said hammer means and said spacer ring for actuating said' hammer inthe opposite direction.

5. A magnetic impulse rapper exposed to adverse ambient conditionscomprising a base having an anvil, a housing attached to said base in afluid tight manner, a

hollow electromagnetic coil enclosed in said housing inl a liuid tightmanner having electric wires attached thereto, a polyester resin casing,around said magnetic coil and interposed between said magnetic coil andsaid housing in a snuggly fit-ting manner and which protects said coilagainst said ambient conditions, means forming a sill at one end of saidresin casing for connecting wires to said magnetic coil through saidhousing and said resin casing, bearing means enclosed in said resincasing, hammer means having anV upper and lower portion enclosed in saidbearing means, said hammer means also having internal means forcommunicating gas from one end of said hammer means to the other,resilient means in operable association with said hammer means, andspacer means interposed between said resilient means and said base,whereby energization of said magnetic coil irnpells said hammer means inone direction to give a hammer-like blow to said anvil and uponde-energization of said magnetic coil said hammer means is actuated inthe other direction so that a rapper is provided which is substantiallyfree from said adverse ambient conditions.

6. A magnetic impulse rapper comprising abase having an anvil, a housingattached to said base in a iiuid tight manner, a hollow coredelectromagnetic coil enclosed inV said housing in aiiuid tight manner, acasing for said coil interposed between said coil and said housing in asnugly fitting manner and which protects said coil against ambientconditions, a bearing means enclosed in said casing made withpolytetrafluoroethylene, a hammer means enclosed in said bearing meansso as to be reciprocatable therein, spacer means located in said bearingmeans in contact with said base, resilient means interposed between saidhammer means and said spacer means, meansr at` tached to said hammermeans and extending intoY said, resilient means responsive toelectromagnetic forces caused' by said electromagnetic coil wherebyenergization of said electromagnetic coil impels said hammer. means inone direction to give a hammer-like blowv to saidjanvil and upon`de-energization of said magnetic coil' said hammer means is actuated intheother direction so that a rapper is provided which is operable insaid adverse ambient conditions and substantially free of maintenance.

7. A magnetic impulse rapper exposed to adverse ambient conditionscomprising a base having an anvil, a housing attached to said base in afluid tight manner, a hollow cored electromagnetic coil enclosed in saidhousing in a iiuid tight manner, a casing for said magnetic coilinterposed between said coil and said housing in a snugly fitting mannerwhereby said coil is substantially unaffected by said ambientconditions, a bearing means enclosed in said casing, a hammer meanshaving an upper and lower portion enclosed in said bearing means so asto be reciprocatable therein, resilient means in operable associationwith said hammer means and a spacer interposed between said' hammermeans and said resilient means, whereby energization of saidelectromagnetic coil impels said hammer means in one direction to give ahammer-like blow to said anvil and upon de-energization of said magneticcoil said hammer means is actuated in the other direction.

8. An electrical precipitator comprising a precipitator shell, anelectrode assembly resiliently supported on said shell, a rapper housingconnected to said electrode assembly and having a hollow portion closedby an anvil portion, said housing also housing an outlet means, a hollowelectromagnetic coil in said hollow portion having outlet wires, amolded polyester resin casing around said coil and so formed that itsoutside diameter tits snugly in said housing and so that said outletwires are connected to said coil through said outlet means in saidhousing without jamming of the wires, means for sealing the hollow boreof said coil with said base andV said housing, a bearing located in saidmagnetic coil, a reciprocatable piston having an upper and lower portionand being positioned in said bearing, said piston having means forcommunicating air from one end of said piston to the other end, saidpiston also having a supporting means thereon extending around itsmid-portion, a polytetrafluoroethylene piston ring riding on saidsupporting means, a spacer ring in said bearing in contact with saidbase, a coil spring interposed between. said polytetrafluoroethylenering and said spacer, whereby energization. of said magnetic coil impelssaid piston in one direction against said anvil of said base tocausevibrations in said electrode assembly and: upon de-energization ofsaid magnetic coil said spring impels said piston in the oppositedirection.

9. An electrostatic precipitator comprising a precipitator shell, anelectrode assembly resiliently supported on said shell, impulsetransmitting means connected through said shell to said electrodeassembly, sealing means between saidl shell and said impulsetransmittingmeans, a rapper housing connected to said impulsetransmitting means and having a hollow portion closed by an anvilportion, a hollow bored electromagnetic coil in said rapper housing, amolded resin casing around said coil formedy so that its outsidediameter tits snugly in saidV housing, means for sealing the hollow boreof said coil with saidv housing, a nonmagnetic bearing in said magneticcoil and coated'with resin having low coefficient of friction, a, pistonreciprocatable in said bearing with an upper and lower portion, saidpiston having means therethrough coinciding with its longitudinal axisfor communicating air from one end of said piston to the other end, saidpiston also having a supporting means thereon extending around itsmiddle portion, a ring ,of resin having a low coefficient of frictionriding on said supporting means, a nonmagnetic spacer ring located insaid bearing in contact` with-said anvil portion of said rapper housing,a coil spring interposed between` said polytetrauoroethylene ring andsaid spacer ring so as to surround said lower portion of said pistonwhereby energization of said magnetic coil impels said piston in onedirection to cause vibrations in said electrode assembly and uponde-energization of said magnetic coil said spring impels said piston inthe opposite direction.

10. An impulse rapper not requiring lubrication and particularly adaptedfor electrical precipitators having adverse ambient conditionscomprising a hollow electromagnetic actuator, a piston in said hollowactuator and actuated thereby, a plastic and metal case enclosing saidhammer in said actuator and completely separating said hammer from saidambient, said piston having rst portion of one diameter and a secondportion of lesser diameter, and a nonmagnetic bearing surface betweensaid actuator and said piston, said bearing surface having thereon acoating of polytetrauoroethylene with a low coecient of friction.

11. An impulse rapper not requiring lubrication and particularly adaptedfor electrical precipitators comprising a hollow electromagneticactuator, a piston in said hollow actuator and actuated in one directionthereby, a resilient means for actuating said piston in the oppositedirection, means enclosing said hammer and resilient means in saidactuator, means for preventing adverse magnetic effects on saidresilient means and a nonmagnetic bearing surface between said actuatorand said piston, said bearing surface having thereon a coating ofpolytetrauoroethylene with a low coeficient of friction.

12. An impulse rapper not requiring lubrication and particularly adaptedfor electrical precipitators comprising a hollow electromagneticactuator, a piston in said hollow actuator and actuated in one directionthereby, a resilient means for actuating said piston in the oppositedirection, means enclosing said hammer and resilient means in saidactuator, means for preventing adverse magnetic effects on saidresilient means, a nonmagnetic bearing surface between said actuator andsaid piston, said bearing surface having thereon a coating ofpolytetrafluoroethylene with a low coeicient of friction, and a meansfor reducing adverse magnetic effects on said resilient means.

References Cited in the ile of this patent UNITED STATES PATENTS1,389,625 Churchward Sept. 6, 1921 1,583,583 Dominguez May 4, 19261,723,607 Dominguez Aug. 6, 1929 1,837,197 Berman Dec. 22, 19311,974,226 Wintermute Sept. 18, 1934 2,268,687 Young Jan. 6, 19422,383,411 Obszarny Aug. 21, 1945 2,462,753 Kyle et al. Feb. 22, 19492,561,355 Fish July 24, 1951 2,574,762 Schell Nov. 13, 1951 2,777,535Hull Jan. l5, 1957 2,909,713 Hambor et al Oct. 20, 1959 FOREIGN PATENTS757,582 Great Britain 1956

